The present invention relates generally to light valves, and, in particular, to high performance photoconductive substrates for use in light valves.
Liquid crystal-based "light valves" or spatial light modulators have found wide application in the fields of image processing, image conversion, and real-time data processing. Light valves have been used, for example, to perform visible to visible image processing, and also to perform image conversion between the visible and infrared spectra Light valves are finding increasing use in the field of adaptive optics, such as for example, to modify the optical properties of laser beams.
The development of and and theory underlying light valve technology is illustrated in such patents as U.S. Pat. No 3,824,002, issued to T. D. Beard on July 16, 1974 and U.S. Pat. No. 4,019,807, issued to D. D. Boswell on Apr. 26, 1977. The basic design of the alternating current (AC) light valve is shown in the Beard patent. An example of a light valve configured to perform visible to infrared image conversion is set forth in U.S. Pat. No. 4,114,991, issued to W. T. Bleha on Sept. 9, 1978. The configuration and operation of the light valve in the hybrid field effect mode, which accomplishes the polarization rotation necessary to effect modulation of an image is discussed in the Boswell patent, as well as in U.S. Pat. No. 4,378,955, issued to W. T. Bleha on Apr. 5, 1983. All of the foregoing patents are owned by the assignee of the present invention.
As discussed in the foregoing patents, the important elements of a light valve are the liquid crystal and the photoconductive substrate. The photoconductive substrate receives the incoming image and controls the polarization rotation of the liquid crystal responsively thereto to accomplish the image conversion or modulation. Prior art light valves, such as those shown in U.S. Pat. Nos. 4,019,807, 4,114,991, as well as in U.S. Pat. Nos. 4,018,509, issued to D. D. Boswell, et al. on Apr. 19, 1977, 4,239,348, issued to Jan Grinberg, et al. on Dec. 16, 1980, and 4,443,064, issued to Jan Grinberg, et al. on Apr. 17, 1984 have typically employed cadmium sulfide or metal oxide semiconductor (MOS)-based photoconductive elements. Simple low-resistivity Schottky silicon structures have also been employed.
These prior photoconductive substrates have several inherent disadvantages. In particular, the previous cadmium sulfide photoconductor structure was based on a polycrystalline (thin-film) photosubstrate which suffered from slow transient response, low sensitivity, non-reproducibility, and non-uniformity.
The prior MOS substrate configuration was limited by premature collapse of the depletion region, the creation of laterally conductive inversion layers which degraded spatial resolution, the use of lattice-damaging processing steps (ion-implantation and thermal oxide growth), and the difficulty associated with flattening a thin processed silicon wafer.
Likewise, the prior Schottky diode-base photo-substrates precluded operation in a symmetrical AC mode. The prior Schottky configuration resulted in the liquid crystal being subjected to direct current, which resulted in electrochemcial degradation of this important layer.
Eliminating the foregoing limitations with the prior art photosubstrates would result in a higher performance light valve. Simplifying the processing of the light valve would also result in a more reliable and economical light valve. Any solution to the foregoing problems with the prior art photosubstrates must, however, maintain the light valve operating in an AC mode, to eliminate the problems caused by DC operation of the liquid crystal.
Accordingly, it is the principal purpose of the present invention to achieve higher performance in a light valve, in terms of improved output uniformity, higher resolution, and higher yield.
It is another purpose of the present invention to simplify the fabrication of light valves.
Yet another purpose of the present invention is to achieve a high performance light valve capable of image processing and conversion in the visible spectral range, as well as between the infrared and visible spectral ranges.